It is known in the prior art to add a reducing agent, such as ammonia for example, to the combustion byproducts within a combustion boiler prior to the combustion byproducts exiting from the combustion boiler in order to reduce the amount of nitrogen oxides remaining in the exhaust stream as the exhaust stream leaves an exit section of the combustion boiler. The reducing agent is generally dispersed in the upper region of the combustion boiler and allowed to react with the combustion byproducts prior to the combustion byproducts exiting via the exit section. One method of applying a reducing agent to the combustion byproducts of a combustion boiler is disclosed in U.S. Pat. No. 4,902,488 while an alternative method is disclosed in U.S. Pat. No. 6,280,695.
As used in the specification and the appending claims, the terms “nitrogen oxides” and “NOx” are used interchangeably to refer to the nitric oxide (NO) and the nitrogen dioxide (NO2) chemical species. Other oxides of nitrogen, such as N2O, N2O3, N2O4 and N2O5, are known but these species are not emitted in significant quantities from stationary combustion sources (except for possible N2O). Thus, while the term “nitrogen oxides” can be used more generally to encompass all binary N—O compounds, it is used herein to refer in particular to the NO and NO2 (e.g., NOx species).
While it is known to apply a reducing agent to the combustion byproducts prior to the combustion byproducts leaving the exit section of the combustion boiler, the prior art methods heretofore have not achieved a maximum reduction in the amount of nitrogen oxides contained within the exhaust stream while also minimizing usage of the reducing agent.
Moreover, while it is generally known, according to U.S. Pat. No. 6,280,695, to inject a reducing agent through overfire air ports into a combustion boiler, this known technique creates reducing agent droplets or particles of a sufficient large size and sufficiently shields the reducing agent droplets in the overfire air stream to delay evaporation and/or gasification of the same so that the lifetime of the reducing agent droplets or particles is greater than the overfire air mixing time with the combustion flue gases. While this technique may reduce somewhat the discharge of nitrogen oxides, a still further reduction in the amount of nitrogen oxides contained within the exhaust stream is required while also minimizing usage of the reducing agent.